Riihi virus was found in Finland in the beginning of December 1993. It's one of the smallest known functioning memory-resident viruses.
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When an infected program is first executed, Riihi becomes memory resident, and infects COM files as they are executed. Riihi does not decrease available DOS memory, and it does not show up in memory maps.
When the virus is resident, it will check every executed program. If the file extension is COM, and the program does not start with the 'M' letter, the virus infects it. The 'M' check is done in order not to infect EXE files that have been renamed to COM.
Infected programs grow by 132 bytes. The virus does not do anything except spread, and it does not contain any texts.
The exact origin of Riihi is not known, but it is suspected to be written in Finland - it caused several large-scale infections in Finland during December 1993 and January 1994.